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GENERAL CHEMISTRY
CHE 101
Lecture 2: Atoms, Molecules and Ions
Course Instructor: HbR
Lecture Plan

The Atomic Theory

The Structure of an Atom

Atomic Number, Mass Number, Isotopes

Molecules and Ions

Compounds
The Atomic Theory
 In
the fifth century B.C.
the Greek philosopher
Democritus said matter
consists of very small
indivisible particles,
named atomos
(meaning indivisible).
3
The Atomic Theory

1783 - Antoine
Lavoisier found that
matter is not created
nor destroyed in a
chemical reaction.
Known as “father of
modern chemistry.”
4
The Atomic Theory
 1808
- English
scientist and school
teacher, John Dalton,
formulated a precise
definition of the
individual building
block of matter that
we call atom.
5
Daltons atomic theory was based on the
following hypotheses :
1. Elements are composed of extremely small particles called
atoms.
2. All atoms of a given element are identical, having the same
size, mass and chemical properties. The atoms of one
element are different from the atoms of all other elements.
3. Compounds are composed of atoms of more than one
element. In any compound, the ratio of the numbers of
atoms of any two of the elements present is either an
integer or a simple fraction.
4. A chemical reaction involves only the separation,
combination, or rearrangement of atoms; it does not result
in their creation or destruction.
6
Dalton’s Atomic Theory (Hypothesis no. 3)
Law of Definite Proportions
(Joseph Proust 1799)
“different samples of the same compound always contain
its constituent elements in the same proportion by mass”
 the ratio of the masses of different elements in a given
compound is fixed,
 the ratio of the atoms in the compound is also constant.
7
Dalton’s Atomic Theory (Hypothesis no. 3)
Law of Multiple Proportions
“if two elements can combine to form more than one compound,
the masses of one element that combine with a fixed mass of
the other element are in ratios of small whole number”
https://www.youtube.com/watch?v=3dWdMqZ2UOU
8
Dalton’s Atomic Theory (Hypothesis no. 4)
16 X
+
8Y
8 X2Y
Law of Conservation of Mass
“matter can be neither created nor destroyed”
9
Structure of the Atom
Atom is the basic unit of an element that can
enter into chemical combination.
 Atom has internal structure and the smaller
particles are called subatomic particleselectron, proton and neutron.

Electron
Coated surface at the
opposite end of the
tube
Cathode Ray Tube Experiment: Discovery of Electron
By J. J. Thomson (1896)
Electrically charged plates
Magnet
Glass tube
Cathode Ray
Cont..

Thomson measured the charge to mass ration
(e/m) of the cathode particle.

In SI units the value is -1.76 X 10 8 coulombs per
gram.

Dutch Physicist H.A. Lorentz named these cathode
particles ‘Electron’.

Millikan’s Measured charge of e- by his oil drop
experiment and the value is -1.60 X 10 -19
coulombs.
Millikan’s e- charge = -1.60x10-19 C
Thomson’s (charge/mass) of e- = -1.76 x 108 C/g
Mass of electron = 9.10 x 10-28 g
13
Structure of the Atom-Electron
◦ An electron can be bound to the nucleus of an
atom by the attractive Coulomb force.
◦ Negatively charged.
◦ A system of one or more electrons bound to a
nucleus is called an atom.
Mass
9.109×10−31kilograms/
5.489×10−4 atomic mass
units (amu)
charge
−1.602×10−19 coulomb
Proton
The Nuclear Model of the Atom:
Rutherford’s Experiment
(+)
ZnS screen
Rutherford’s postulation on nuclear
atomic structure:



Atom has a tiny dense central core or the
nucleus which contains practically the entire
mass of the atom, leaving the rest if the atom
almost empty.
The entire positive charge of the atom is
located on the nucleus, while electrons were
distributed in vacant space around it.
The electrons were moving in orbits or closed
circular paths around the nucleus like planets
around the sun.
Proton
As an atom is neutral and contains (-)
charged electrons, it must contain equal
amount of (+)ly charged particles.
 Proton is positively charged particles and
remains in the nucleus of the atom.

Mass
1.67262 x 10-24 g (1840 times
the mass of e-)
charge 1.602×10−19 coulomb
Neutrons
In 1932 Sir James Chadwick discovered neutron.
 A stream of alpha particles were directed at a beryllium
target.
 He found that a new particle were ejected who has a
property like γ ray. It has almost the same mass as the
proton and has no charge.

?
Structure of the Atom-Neutron
Third type of subatomic particle.
 Electronically neutral
 Mass is slightly higher than proton
 Remains with proton in the nucleus.

Mass
1.67493 x 10-24
charge 0
Atomic Number
Atomic number (Z) is the number of
protons in the nucleus of each atom of an
element.
 In a neutral atom,

number of electrons = number of protons
Chemical identity of an atom is the ‘atomic
number’.
 Example:

atomic number of fluorine is 9. It means it has,
9 protons. /
proton number 9 indicates that the atom is
‘fluorine’ .
Mass Number
Mass number (A) is the total number of
neutrons and protons in the nucleus of an
atom of an element.
 Mass number = numbers of protons+
numbers of neutrons
= atomic number + numbers of neutrons
 Numbers of neutrons= ?

* Atomic number, mass number and numbers of protons
and neutrons all must be positive integers(whole
numbers)
Isotopes
Atoms that have same atomic number but
different mass number.
 Most elements have two or more isotopes.
 Example:

Mass number
A
X
:Element symbol
Atomic number
Z
1H
1
3H(T)
1
2H(D)
1
/
235U
92
238U
92
The Isotopes of Hydrogen
Hydrogen
Deuterium
Tritium
23
Counting Protons, Neutrons & Electrons
14
How many protons, neutrons, and electrons are in C ?
6
How many protons, neutrons, and electrons are in11C ?
6
24
1. Give the number of proton, neutrons and
electrons in each of the following species:
17
8
O,
199
80
63 Cu
29
Hg,
200
80
Hg,
Molecules

Molecules: is an aggregate of at least two atoms in a
definite arrangement held together by chemical
forces( bonds).

Molecules can contain atoms of same elements or
different elements.

They should join in a fixed ratio.

Electrically neutral like atom.
Molecules
Of all the elements, only the six noble gases in Group
8A exist in nature as single atoms, called monatomic
gases.
Most matter is composed of molecules or ions
formed by atoms.
A diatomic molecule contains only two atoms.
H2, N2, O2, Br2, HCl, CO
diatomic elements
A polyatomic molecule contains more than two
atoms.
O3, H2O, NH3, CH4
H2
H2 O
NH3
CH4
Ions

An Ion is an atom or a group of atoms that has
a net positive or negative charge.

The number of “Proton” in the nucleus
remains same during chemical reaction.

“electrons” are lost/gained during a reaction.
◦ The loss of 1/more electrons from a neutral atom
results “cation” : an ion with (+) charge .
◦ The gain of 1/more electrons from a neutral atom
results “anion”: an ion with (-) charge.
Ions
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
30
Ions
NaCl is an ionic compound, because it is composed of cation (Na+) and Anion
(Cl-)
Monatomic Ions & Polyatomic Ions
A monatomic ion contains only one atom
Na+, Cl-, Ca2+, O2-, Al3+, N3-
A polyatomic ion, also known as a molecular ion, is
a charged chemical species (ion) composed of two or
more atoms covalently bonded or of a metal complex
that can be considered to be acting as a single unit.
OH-, CN-, NH4+, NO3-
Counting Protons, Neutrons & Electrons
How many protons, neutrons and electrons are in
13 protons,
14 neutrons,
27 3+
13
34
44 neutrons,
?
10 (13 – 3) electrons
How many protons, neutrons and electrons are in 78
34 protons,
Al
Se 2-
?
36 (34 + 2) electrons
33
Compounds

A chemical compound is a pure chemical
substance consisting of two or more
different chemical elements that can be separated
into simpler substances by chemical reactions.

Chemical compounds have a unique and
defined chemical structure; they consist of a fixed
ratio of atoms.

They are held together in a defined spatial
arrangement by chemical bonds.
Compounds
Characteristic properties of compounds:

Elements in a compound are present in a definite
proportion
Example- 2 atoms of hydrogen + 1 atom of oxygen
becomes 1 molecule of compound-water.

Compounds have a definite set of properties
Elements that comprise a compound do not retain
their original properties.
Naming Compounds
Organic compounds
◦ Contain carbon.
◦ Usually in combination
with elements such as
H, O, N, and S.
◦ Can contain functional
groups.
Inorganic compounds
◦ All other compounds are
classified as inorganic
compounds.
◦ CO, CO2, CS2, and
compounds containing
CN−, CO32−, and HCO3−
groups.
Naming Compounds
Inorganic compounds are divided into four categories:

Ionic compounds: often metal + non metals. e.g. Mg(OH)2,
KNO3, NaCl etc.

Molecular compounds:nonmetal + nonmetal/metaloids. e.g.
H2O

Acids and bases:H+ / proton donar/acceptor: acids and
bases respectively. e.g: HCl, NaOH.

Hydrates: specific number of water molecules will be
attached. e.g. BaCl2•2H2O : barium chloride dihydrate.
LiCl•H2O : lithium chloride monohydrate
CuSO4•5H2O
CuSO4
Thank You
Practice mathematical related problems from Raymond Chang 9th/10th Ed.